Privacy system



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A. c. DlKiEsoN PRIVACY SYSTEM Filed Julyv 19, 1941 EN you /NVENTOR y AC/CK/ESN Patented Sept. l0, 1946 JNET STATES PATENT CFHCE PRIVACY SYSTEMApplication July 19, 1941, Serial No. 403,098

8 Claims.

This invention relates to signal Wave transmission systems andparticularly to circuits for rendering signal transmission over suchsystems unintelligible to unauthorized listeners.

The invention is particularly applicable to secrecy or privacy systemsof the band-splitting and scrambling type, such as disclosed, forexample, in my U. S. Patent 2,132,205, issued October 4, 1938. In thesystem of that patent the whole frequency band of the speech or othersignal wave to be transmitted is shifted by modulators in separatecircuits to respectively different higher positions in the frequencyspectrum with a certain frequency ran-ge within the frequency limits ofthe original signal wave common to all of the shifted bands, this commonfrequency subband being selected by a lter in each circuit. The selectedsubbands are interchanged, with or Without inversion of the frequenciesin certain of the interchanged subbands, among the several circuits inaccordance with selected secret combinations which are changed from timeto time. Other modulators in the respective circuits are used to shiftthe interchanged subbands downwards to adjacent positions in thefrequency spectrum embracing a total frequency range substantially equalto that of the original signal frequency band. The subbands in thelatter shifted positions are superposed in a common circuit andtransmitted together over the line or other transmission medium to areceiving station where by apparatus which is the same as used at thetransmitting station but operating in reverse order they are transformedto reproduce the original intelligible signal wave.

It is known that the intelligibility of a subband divided out of aspeech band depends not only on the frequency width of the subband butalso on its location in the speech band. If the important band offrequencies, Z50-3000 cycles, in a speech wave is divided into aplurality of, say five, subbands, A to E, of equal width (550 cycles),it has been determined by listening tests that the lower frequencysubband (250-800 cycles), which will be referred to hereinafter as the Asubband, has substantially 100 per cent sentence intelligibility, andthat the sentence intelligibility for the other subbands rapidlydecreases as the position in the frequency spectrum becomes higher.

The efforts of an ordinary eavesdropper making use of a heterodynecarrier of adjustable frequency to nd the A subband and thus to listenin on a conversation over a system employing a band-splitting andscramblingv privacy system such as described above, may be circumventedby proper choice of the interchanged subband combinations so that, if hetunes the A subband into its proper location, one of the other subbandswill be superimposed and, therefore, masking. However, a more determinedand skillful eavesdropper equipped with more complicated apparatus maybe able to find the A subband and pick it out free of any maskingfrequencies, One way of circumventing such an eavesdropper is to employnarrower subbands and to have their frequency positions switched morefrequently. If the subbands are kept of equal width, it would benecessary to go to a ten subband system to get any appreciableimprovement in privacy by such means, which would double the size andcost of present systems employing ve subbands.

An object of the present invention is to increase the privacy of privacysystems of the type described above without increasing the number ofsubbands switched and with the addition of comparatively little andeconomical apparatus. This object is attained in accordance with theinvention by arrangements operating to introduce additional distortionin the particular subband having the greatest amount of sentenceintelligibility while maintaining the general scheme of frequencytransformation and switching described above.

The various objects and features of the invention will be understoodfrom the following detailed description when read in conjunction withthe single figure of the drawing showing diagrammatically one terminalof a two-way telephone system employing a bilateral privacy device ofthe band-splitting and scrambling type modified in accordance with oneembodiment of the invention.

The privacy or secrecy system of the invention will be described asapplied to a voice-frequency telephone system although its basicprinciples are applicable to other types of signaling systems and tohigher frequencies.

In the two-way telephone terminal of the drawing, the bilateral privacydevice is shown connected between portions of the one-way transmissioncircuit TE transmitting in the direction from west to east and portionsof the one-way transmission circuit TW transmitting in the directionfrom east to west, so as to allow transmission over each of thesetransmission circuits through the privacy device while maintaining aconjugate relationship between the two circuits.

The privacy device employs five band-splitting branches or channelsidentified as A to E, respectively. At the west end of the circuit theincom- La' ing portion of the one-way circuit TE and the outgoingportion of the one-way circuit TW are coupled by the hybrid coil H1 andassociated balancing network N1 to the two circuits I and 2. The circuitl is coupled by hybrid coil H2 and associated network N2 with channels Aand B. The circuit 2 is coupled by hybrid coil H3 and associatedbalancing network N3 with the channel C and the circuit 3; and thecircuit 3 is coupled by hybrid coil H4 and associated balancing networkN4 with the channels D and E.

At the east end of the circuit, the outgoing portion of circuit TE andthe incoming portion of circuit TW are coupled by hybrid coil H5 andassociated balancing network N5 with the circuits 4 and 5. The circuit iis coupled by hybrid coil He and associated balancing network N6 to theeast ends of channels A and B. The circuit 5 is coupled by hybrid coilH7 and associated balancing network N1 with the east end of channel Cand a circuit 6. The circuit 6 is coupled by hybrid coil H8 andassociated balancing network Na with the east ends of channels D and E.

The channels B to D include in order reading from west to east: a lterF1; a modulator M1; a filter F2; two parallel circuits respectivelyincluding an inverter I and an attenuation pad P3 providing a circuitloss equal to that of the inverter I, having their east ends coupledwith the east portion of the same channels B to D, respectively, byequivalent hybrid coils Hs and associated balancing networks N9; asecond lter F2; a second modulator M2; and a second filter F1. Thechannel E contains the same elements connected inthe same order as thechannels B to D except that an attenuation pad P1 and an at tenuationpad P2, respectively, providing a circuit loss equal to that of themodulator M1 and the rst ilter F1 and the modulator M2 and second lterF1 in the other channels are substituted for the latter modulators andfilters.

The channel A differs from the channels B to D in that its input portioncomprises two branch circuits A1 and A2 having their west ends connectedin parallel to the circuit leading to the hybrid coil H2 and their eastends coupled by hybrid coil H10 and associated balancing network N10 tothe west end of the filter F2. The branch A1 .includes a lter Fs and amodulator M1 and the branch A includes a filter Fi and a modulator M1,substituted in place of the rst lter F1 and modulator M1 employed in thesingle circuit of each of the channels B to D.

Associated with each ofv the channels A to E at a po-int between thefirst filter F2 and the parallel inverter and attenuation Dadbranches infront of the second filter F2 is a switching circuit indicated by a boxlabeled SW, the function of which is to Vconnect the first lter F2 ofeach channel with the inverter I or the attenuation pad P3 of the sameor any of the other channels toscramble up the frequency subbandsselected by that lter in theseveral channels in accordance with any o-neof a number of selected secret combinations, with or without inversionof those frequencies within the individual subbands depending on whetherthe filter F2is connected to an inverter or an attenuating pad. Theswitching arrangements SW may be of the type illustrated in Chesnut etal. Patent 1,829,783, issued November 3, 1931, and may be synchronizedwith the similar switching arrangement at the other terminal of thesystem in the manner described in that patent.

Each of the filters F1 in the four channels A to D are identicallow-pass lters passing the same speech frequency range 0-2450 cycles persecond, and each of the filters F2 in the channels A to E are identicalband-pass filters passing the frequency range, 2450-3000 cycles persecond. The filter F3 in branch A1 of channel |A is a bandpass filterpassing the frequency range 250-550 cycles, and the filter F4 in thebranch A2 of channel A is a band-pass filter passing the frequency range550-800 cycles.

Each of the modulators M1, M2 in the channels A to E are bilateralmodulators, preferably o-f the known double-balanced copper-oxide type.The modulator M1 in branch A2 of channel A is supplied witha carrierfrequency of 3250cycles; the modulator M1 in branch A1 of channel A issupplied with a carrier frequency of 1900 cycles; and the modulator M1in channels B to D is supplied with the carrier frequencies 3800, 4350and 4900 cycles, respectively. The modulators M2 in the channels A to Dare supplied with 2, carrier frequency of 3250, 3800, 4350 and 4900cycles, respectively. The inverters I in the inverter branches of thechannels A to E are supplied with a carrier frequency of 5450 cycles.All of these carrier frequencies may be supplied by a single tonegenerator of the inductor type, such as is described in H. M. StollerPatent 1,637,233, issued October 9, 1928.

For the case of speech wave transmission through the privacy device fromwest to east the operation is as follows. Let it be assumed that aspeech wave having a frequency band of 0-3000 cycles is received at thewest end of the privacy device over the circuit TE. The energy of thiswave is divided into two parts by the hybrid coil H1, which pass intothe circuits l and 2, respectively. The portion of the waves in circuitI is divided into two energy portions by hybrid coil H2 whichrespectively pass into channel A and channel B. The portion of thespeech energy in circuitA 2 is divided into two parts by hybrid coil H3,which pass respectively into channel C and circuit 3. The speech energyin circuit 3 is divided by hybrid coil Hi into channels D and E. Thusthe incoming speech wave is divided into ve energy portions respectivelytransmitted into the iive privacy channels .A to E.

The speech energy in channel A in the frequency range 250-800 cycles,which, as stated above, is the range which has the greatest amount ofsentence intelligibility, is divided into two equal portions offrequency range 250-550 cycles and 550-800 cycles by the parallel ltersFs and F4 in the branches A1 and A2 of channel A passing theserespective frequency bands.

The frequency range 250-550 cycles is modulated with a carrier of 3250cycles in the modulator Mi in branch A1, yielding a lower side-band of3000-2700 cycles, and the frequency range of 55o-800 cycles is modulatedwith a carrier of 1900 cycles, giving an upper sideband of 2450-2700cycles. As the outputs of the two modulators M1 in the branches A1 andA2 are coupled through hybrid coil Hio and associated balancing networkN10 to the input of the rst lter F2 in the channel, passing the samefrequency range, 2450-3000 cycles, Vas the corresponding lter F2, in theoutput 0f the modulator M1 in the channels B to D and in the output ofattenuator D, in channel E, the A subband range is placed in the commonband lter frequency range so that it can be switched in the usual mannerbetween the outgoing portions of the channels A to E by the switchingdevice SW, but in thev process, the A subband has been cut approximatelyin half and the frequencies in the two halves inverted with respect toeach other.

Now if an eavesdropper uses a band filter of 550 cycles in width andtunes in the lower part of the A subband, he will receive the Z50-550cycle part in its right location, with the 550-800 cycle part adjacentbut with its frequencies inverted. Thus, in addition to the reduction inintelligibility due to the masking provided by the adjacent invertedspeech band, the intelligibility will be further substantially decreaseddue to the inversion of the frequencies in the two halves of the A band.

The operation of the remaining portion of the system of the drawing willbe the same as described in my aforementioned patent for thecorrespondingly identified circuit elements and need not be repeatedhere.

When speech signals are being transmitted in the direction from east towest, the operation will be the reverse of that described with thefrequencies passed through the left-hand 2450-3000 cycle filter F2, inthe direction from east to west in the channel A divided by hybrid coilHio and associated balancing network N into two equal energy portions inthe branch circuits A1 and A2. These are respectively modulated withcarriers of 3250 and 1900 cycles in the modulators M1 in the twobranches to reproduce the original 250-550 cycle and 550-800 cyclesubbands with their frequencies in proper order which are selected byfilters F3 and F4, respectively, are combined in the outputs of thesefilters and pass through the hybrid coils H2 and H1 to the outgoingportion of the transmission circuit TW at the east terminal where theyare combined with the other frequencies transmitted in the east-towestdirection over the channels B to E in the manner described in myaforementioned patent, to reproduce the original signal waves with allfrequencies in proper intelligible order.

It will be apparent that the intelligibility of the transmitted signalsto an eavesdroppe'r may be further decreased if the B subband which isnext in sentence intelligibility also be split into two halves and thefrequencies in the two halves inverted in the manner which has beendescribed for the subband A. Other modifications of the circuitsillustrated and described which are within the spirit and scope of theinvention will -be apparent to persons skilled in the art.

What is claimed is:

1. The method of transmitting a message with privacy which consists inderiving from a message wave having a band of frequency components, aplurality of frequency subbands having the same frequency limits but thefrequencies in each subband representing respectively differentfrequency ranges in the original message band, and with the frequenciesin the two halves of the subband corresponding t0 that frequency rangein the original message band having the greatest amount ofintelligibility inverted with respect to each other, interchanging saidsubbands among several circuits, and transmitting the interchangedsubbands.

2. The method of making a transmitted message unintelligible whichconsists in transforming a message wave of a band of frequencycomponents into a plurality of relatively narrow subbands each havingthe same frequency limits but the frequencies in each subbandrepresenting respectively different frequencies in different frequencyranges of the original message band, separately selecting from theoriginal message band a subband having a, relatively large amount ofintelligibility, transforming the latter subband to produce anothersubband having the same frequency limits as each of said plurality ofsubbands but y with the frequencies in two approximately equal portionsinverted with respect to each other, interchanging said plurality ofsubbands and said other subband among several circuits, and transmittingthe interchanged subbands.

3. In a speech privacy system, means to derive in a plurality ofseparate circuits a plurality of narrow bands of waves of equal bandwidth from different portions of the speech frequency band, each suchnarrow band being practically unintelligible by itself but one of saidbands carrying a larger amount of intelligibility than the others, meansto shift said narrow bands of waves about among themselves and totransmit them to a distance in such shifted order to provide privacy,and means to increase the privacy comprising means to subdivide said oneband having the larger amount of intelligibility into frequency subbandsand the rearrange the frequency relations of certain of the subbands toreduce their intelligibility.

fl. A privacy system comprising means for transforming a message wave ofa band of frequency components into a plurality of frequency subbandseach having the same frequency limits but the frequencies in eachsubband representing those in respectively different frequency ranges ofthe original message band and with the frequencies in the two halves ofthe one of said subbands having the greatest amount of intelligi; bilityinverted with respect to each other, means for interchanging all of saidsubbands including said one subband among an equal number of circuits,and means for superposing the interchanged subbands in a common circuitfor trans.4 mission.

5. A privacy device comprising a plurality of circuits each suppliedwith the whole frequency band of a signal wave to be made secret,modulating and ltering means in one of said circuits for transformingthe subband of the supplied signal wave having a relatively large amountof intelligibility compared with the other subbands into anothernarrower subband in a predetermined position in the frequency spectrumand with the frequencies in the two halves of the latter subbandinverted with respect to each other, modulating and filtering means inthe other l circuits for respectively transforming the supplied signalband into a plurality of subbands each having the same frequency limitsas said other subband, the frequencies in the several subbandsrepresenting respectively different frequency ranges in the originalsignal band, means for in.. terchanging all of the transformed subbandsincluding said other subband among said circuits in a predeterminedarrangement, and means for combining the interchanged subbands in acommon circuit for transmission,

6. A privacy system comprising a plurality of circuits each suppliedwith the same band of frequency components representing a message to bemade secret, means in each circuit for shifting the supplied band to adifferent respective position in the frequency spectrum such that eachshifted band has a subband of frequencies in common with each of theother shifted bands within the frequency range of the original messageband, means in each circuit for selecting the common subband, means forinterchanging thev selected sulobands` among the several circuits, withor without inversion of the frequencies within the interchangedsubbands, means in each circuit for shifting the interchanged subbandsso that all interchanged hands have adjacent re.- spectively differentfrequency positions embracing substantially the same total frequencyrange as the original message band, alcommoncircuit, means forimpressing the interchanged subbands in their final shiftedfrequencypositions superimposed on one another upon said common circuitfor transmission, and means for inverting with respect to each other thefrequencies in two substantially equal portions of the one of the commonsubbands in one circuit representing the frequency range in the originalmessage band having the maximum amount of intelligibility, prior toselecting it and interchanging it with the other subbands.

7 The privacy system of claim 6, in which the last-mentioned meanscomprises two parallel branches in the input of said one circuit, eachincluding means for selecting a different half of said one frequencyrange in the supplied message band, having said maximum amount ofintelligibility, modulating means in each branch for individuallymodulating the selected half of said one frequency range with a wave ofselected fre quency so that' the modulation products in the two branchesrespectively include an upper and lowervmessage side-band ofrespectively inverted frequencies which together comprise said commonsubband of frequencies and means for im.- pressing said upper and saidlower side-band in superposition. on said common subband. selectingmeans in said one circuit.

8. A privacy systemcomprising a wave transmissicnmedium connectingstation, thetransmitting stations comprising a plurality of parallelcircuits each supplied with the same band of frequency componentsrepresenting a message to be transmitted, means in each circuit forshifting the supplied band to a different respective position in thefrequency spectrum such that each shifted band has a subband offrequencies in common with each of the other shifted bands, means ineach circuit for selecting the common subband, means for interchangingthe selected subbands lamong the several circuits in a predeterminedarrangement, means in each circuit for shifting the interchangedsubbands so that the interchanged subbands have adjacent respectivelydifferent positions inthe frequency spectrum embracing substantially thesame total frequencyrange as the original message band, means fortransmitting the interchanged subbands in their final shifted frequencypositions superim* posed on one another to said medium, means forrearranging the frequency-relations of different parts of the one of theselected subbands having the greatest amount of intelligibility prior tointerchanging the selected subbands among the several circuits tofurther reduce the. intelligibility of the message wave, said receivingstation comprising the same circuits and means as the transmittingstation as. recited above but connected in reverse order for reproducingthe original message wave from the wave received thereby from saidmedium.

ALTON C. DICKIE'SON.

